'Stern' (ship) is Old Norse for 'steering' — from 'stjorn' (rudder). A Viking loanword for the rear.
The rear part of a ship or boat, opposite the bow; the after end of a vessel.
From Old Norse 'stjórn' (a steering, a rudder, the stern of a ship), from 'stýra' (to steer), from Proto-Germanic *steurijaną (to steer), from PIE *steh₂- (to stand, to set in place) or possibly *stew- (to push, to knock). The connection between 'steering' and 'stern' is direct: the stern was where the steering mechanism was located. Note: this is a different word from the adjective 'stern' (severe, strict), which has a separate Germanic etymology
The noun 'stern' (back of a ship) and the adjective 'stern' (severe, unyielding) are unrelated words that happen to be spelled the same. The nautical stern comes from Norse 'stjórn' (steering); the adjective comes from Old English 'styrne' (severe), possibly related to 'stare.' Their identical modern spelling is a coincidence of sound change